Light curtains are commonly employed in a variety of industrial applications to detect the intrusion of objects, such as hands or feet of personnel, into or around a prescribed area or zone of protection. Typical applications for light curtains include provide operator protection around machinery such punch presses, brakes, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. Conventional light curtain systems employ invisible pulsed infrared light beams which project across a plane along the zone to be protected. Unintended intrusion of the light beams by an object, such as the operator's hand, is sensed so that a control circuit triggers a warning signal, shuts the machinery down, or otherwise safeguards the area.
Heretofore light curtain systems have been provided with transmitter and receiver heads arranged to pulse the beams across a single plane. The heads are typically mounted so that the plane is positioned between the human operator and the machine, such as in the front of a punch press. Additional light curtain systems with separate transmitter and receiver heads are required where it is desired to provide protection to other areas of the machine, such as along either or both sides. Because light curtain systems are expensive, this leads to increased cost of the installation.
In certain cases it has been found that machine operators, in an effort to increase their productivity, attempt to foil the protection of the light curtains. For example, in the case of a single light curtain projected in a vertical plane between the operator and machine, the operator may attempt to reach under or around the plane of the curtain to feed the work pieces at a greater rate through the machine. With the light curtain system foiled in this manner, serious injury can result.
Certain light curtains heretofore available have included control systems by which selected beams can be electronically disabled to permit the sensing area to be penetrated by expected tooling or work pieces. An example is a metal forming machine in which the formed work piece is fed from the machine out through an electronically masked-off portion of the sensing area. The remaining portion of the sensing area continues to operate for sensing intrusion of the operator's hands or other objects. However, in these conventional systems there are shadows of unprotected areas on opposite sides of the masked-off portion of the curtain. As a result, injury or damage can occur from intrusion of a hand or other object into the unprotected area.